In addition to an overload relay, which identifies overloading of a motor on the basis of the current drawn by the motor, thermistor protective circuits are also used for protection against thermal motor overloading, and these circuits detect and evaluate the temperature of a motor winding. Various temperature detector elements are used for this purpose, for example temperature sensors, in particular PTC thermistors (PTC: positive temperature coefficient), PT100 sensors and KTY sensors, whose electrical resistance varies as a function of the detected temperature. In particular, the resistance of a temperature detector element such as this is in a first range for as long as the motor winding is in a temperature range in which there are no problems. The second resistance range, whose resistance values are greater, for example, than those of the first resistance range, then corresponds to a temperature above a temperature threshold value.
Furthermore, so-called thermo-click elements, which are also referred to as temperature monitors, can also be used as temperature detector elements. These are generally represented by simple switches which are normally designed using a bimetallic strip, which is closed in the first temperature range and is opened in a second temperature range, that is to say above the temperature threshold value. This means that the resistance changes from virtually 0Ω to an infinite resistance.
In the case of the first-mentioned temperature detector elements, the temperature sensors, overloading of the motor is evaluated by way of the resistance change of the temperature detector elements within predefined value ranges. In contrast, in its normal field of application, the thermo-click element opens the circuit through a motor winding on reaching the temperature threshold value. In contrast, temperature sensors are evaluated by means of a protective circuit which essentially determines the electrical resistance of the relevant temperature detector elements and emits a message signal to a switch, to an evaluation circuit or the like, thus, for example, initiating a shutdown of the appliance or starting some other predetermined function when a thermal overload is identified as a function of the measured temperature.
Protective circuits such as these are frequently likewise provided with short-circuit detectors, which likewise result in the corresponding message signal being emitted and thus, for example, initiate a shutdown of the appliance to be protected when a short circuit occurs in the sensor circuit. Without this short-circuit identification, it is no longer possible to identify a thermal overload when a conductor short-circuit occurs, since it is no longer possible to identify an increase in the resistance of the temperature sensor. Since short-circuit identification is necessary in order to obtain a specific license, for example ATEX (that is to say the appliance can be used for protection of motors in explosion-hazard areas), it is desirable to use a protective circuit such as this with short-circuit identification.
If one wishes to use a protective circuit such as this for temperature sensors of the abovementioned type, for example PTC thermistors, PT100 sensors and KTY sensors, as well as for thermo-click elements, the closed state of the thermo-click element is identified in the normal state, that is to say at temperatures in the first temperature range, as a short circuit through the protective circuit, which then initiates a shutdown of the appliance.
In consequence, until now, it has not been possible to use a protective circuit which is provided for temperature sensors in conjunction with a thermo-click element at the same time.
Until now, only protective circuits which evaluate temperature sensors without short-circuit identification have been known, which are therefore also suitable for evaluation of thermo-click elements, or circuits for thermistors which have short-circuit identification and are therefore not suitable for evaluation of thermo-click elements. In addition, evaluation circuits are already known in which it is possible to use adjustment parameters or configuration to choose between operation with short-circuit identification and operation without short-circuit identification.